It is known that a typical Local Area Network (LAN) can use routing bridges to route data among stations connected to the LAN. The routing bridges receive and transmit data packet-by-packet at data link layer level. There are several protocols to define data routing at data link layer, including: source routing defined by IEEE 802.5, transparent routing defined by IEEE 802.1D, and source-transparent routing which is a are facto industry protocol.
Bridges build according to source routing, transparent routing and source-transparent routing are referred as source routing bridge (SR-bridge), transparent routing bridge (TB-bridge) and source-transparent bridge (SR-TB-bridge), respectively. Analogously, LANs using source routing and transparent routing are referred as source routing bridged LAN (SR-LAN) (e.g., a Token-Ring LAN) and transparent routing bridged LAN (TB-LAN), respectively.
Source muting requires each of the packets to be transmitted to contain route information inserted by a source end station. An SR-bridge uses the routing information to determine whether it should forward the packets, and to what LAN-segment the packets should be sent. A LAN-segment is a part of a LAN where end stations can communicate directly with each other via the LAN media, without the need for an intermediate link, such as a bridge or a router. For example, in a Token-Ring LAN, a LAN-segment is a ring. In an Ethernet LAN, a LAN-segment is a cable including components like "repeaters" or hubs.
According to source routing protocol, a source end station initially invokes a route determination procedure to "find" a route to a destination end station. The source end station transmits copies of a special kind of packet and sends a copy to each possible path. Each copy collects route information while it is being transmitted over LAN-segments and bridges.
When a source end station finds a route to a destination end station, it keeps the routing information so that the information can be inserted for subsequent packets to the same destination end station.
FIG. 1 depicts a typical configuration for an SR-LAN, where stations A and B are connected with each other through SR-bridges 1-2 and SR-segments 1-3. By transmitting copies of a special kind of packet and sending a copy to each possible path, station A finds the route to station B as being: SR-segment 1, SR-bridge 1, SR-segment 2, SR-bridge 2, SR-segment 3, and station B. When station A wants to deliver packets to station B, it inserts this routing information into the packets designated for station B.
When station B responds to a message from to station A, station B inserts the same routing information, as it reads from the packet(s) from station A, in reverse order. Whether station B takes this routing information from only the first packet sent by station A, or it continuously updates the routing information from all packets from station A is implementation dependent. In contrast to source routing, transparent routing does not need the route information as required by source routing.
To accommodate the packets with and without the routing information, an SR-TB-bridge is used to connect an SR-LAN to a TB-LAN. Because the packets on a TB-LAN do not contain the muting information required for source routing, an SR-TB-bridge should perform be capable of: (1) responding to any route determination procedure initiated by a station on the SR-LAN, to find routing information for a station on the TB LAN, (2) maintaining the routing information to end stations on the SR-LAN in its location data base, (3) adding the routing information to the packets transmitted by end stations on the TB-LAN and destined for end stations on the SR-LAN based on its location data base, and (4) removing the routing information from the packets transmitted by the end stations on the SR-LAN and destined for the end stations on the TB-LAN.
Conceptually, this process can be regarded as if the SR-TB-bridge handles the source routing procedures on behalf of the stations on the TB-LAN.
A well known commercially available SR-TB-bridge is IBM 8209 LAN Bridge.
FIG. 2 depicts a typical configuration where an SR-LAN is connected to a TB-LAN through an SR-TB-bridge. In FIG. 2, stations A and B are connected with each other through SR-bridges 1-2, SR-TB-bridge 3, SR-segments 1-3, and TB-segment 4.
In FIG. 2, if mobile station A transmits the packet destined for station B, this packet is received by SR-TB-bridge 3 on TB-segment 4. If SR-TB-bridge 3 has the routing information to station B stored in its location data base, it will insert this routing information in the packet and forward the packet to station B via the SR-LAN. If the route information to station B is not in its location data base, SR-TB-bridge 3 will invoke a route determination procedure to generate the routing information. The routing information in this example is: SR-segment 1, SR-bridge 1, SR-segment 2, SR-bridge 2, and SR-segment 3.
The problem for a conventional SR-LAN is that it can not properly transmit data in a roaming environment, where a station that is roaming dynamically changes access points on the SR-LAN. The primary reason for this problem is that the muting information is "semi-static" and kept in the correspondent stations the roaming station is communicating with. "Semi-static" means that the information is not constantly, dynamically; updated. It is determined at a certain point in time, and used for some period of time, depending on situations, higher level procedures, implementation, and usage.
U.S. Pat. No. 5,371,738 entitled WIRELESS LOCAL AREA NETWORK SYSTEM WITH MOBILE STATION HANDOVER by H. Moelard et al. discloses a method of handling station roaming in a TB-LAN.